The present invention relates generally to permanent magnet arrangements for equipment used to conduct magnetic resonance imaging (MRI) examinations and, in particular, to a magnet arrangement which uses a backplate to increase magnetic field strength and a field adjusting apparatus built into the backplate which allows magnetically permeable shim blocks within the backplate to be individually adjusted to strengthen and align the magnetic field. This application claims priority from U.S. patent application serial No. 60/177,958 filed Jan. 25, 2000 and entitled xe2x80x9cField Adjusting Mechanisms and Methods For Permanent Magnet Arrangement With Backplatexe2x80x9d.
Apparatus used for MRI diagnostic procedures require the use of large magnets to create the strong, uniform magnetic field required for accurate test results. An MRI unit must be of a size and configuration to enable a patient to place all or a substantial part of his or her body into that portion of the magnetic field found to be the strongest and most uniform.
A typical MRI apparatus features an opposed pair of magnet groupings supported by a generally C-shaped frame designed to hold the groupings in face-to-face parallel configuration. An air gap or space is defined between the magnet groupings where the patient is positioned. Each of the frame legs and cross members is made from a highly permeable ferromagnetic material to complete a circuit for the magnetic flux to travel from one magnet grouping across the air gap to the remaining grouping.
Because the magnet groupings are quite heavy, it is necessary to construct the frame of sufficiently massive members to limit deflection of the frame not only by the weight of the magnets but the weight combined with the attractive force the magnet groupings exert on each other. The weight of the magnet groupings and the massiveness of the frames needed to support them can make MRI units costly and difficult to construct and inconvenient to move.
MRI units and the magnet groupings used in them are well represented in the prior art.
U.S. Pat. No. 5,623,241 (Minkoff) teaches and describes a permanent magnetic structure comprising a C-shaped frame which supports two opposed magnet groupings at the open end of the frame legs, thereby forming an air gap. The groupings in Minkoff are single-piece, permanent magnets arranged in parallel, face-to-face relationship, each magnet having a pole piece positioned on its outer face.
U.S. Pat. Nos. 4,943,774 and 5,134,374 (Breneman, et al.) teach and describe various magnetic field control apparatus. Both patents feature magnet groupings consisting of a permanent magnetic pole supported on a rear frame and having a pole face formed from ferrous material. Breneman, et al ""774 utilizes a number of ferrous segments positioned on an annular member attached to one pole face, with the segments being radially repositionable to adjust the magnetic field strength and orientation. Breneman, et al ""374 uses a series of segments applied directly to the pole faces to adjust the magnetic field.
U.S. Pat. No. 5,194,810 (Breneman, et al.) teaches and describes a superconducting MRI magnet with magnetic flux field homogeneity control which uses radially positioned circular segments as shims to adjust the magnetic field created between two magnet groupings.
U.S. Pat. No. 5,332,971 (Aubert) teaches and describes a permanent magnet for nuclear magnetic resonance imaging equipment utilizing a number of magnetic blocks arranged into concentric rings to produce a homogeneous magnetic field.
U.S. Pat. No. 5,659,250 (Domigan, et al.) teaches and describes a fill brick construction of magnet assembly having a central bore in which a plurality of individually magnetized bricks are arranged in elliptically shaped sections to create a bore within which a patient can be placed for MRI examination. The individual bricks are formed as parallelepiped and the field is adjusted by the shapes of the groupings of bricks used.
U.S. Pat. No. 4,998,084 (Alff) teaches and describes a multipolar magnetic ring consisting of two rings having magnetic segments formed about their inner peripheries which interengage to form a single ring.
U.S. Pat. No. 4,734,253 (Sato, et al.) teaches and describes a method for the preparation of sintered magnets from Fe-B rare earth alloy with the sintered magnetic segments arranged to form a circular ring.
U.S. Pat. No. 4,538,130 (Gluckenstern, et al.) teaches and describes a tunable segmented ring magnet and method of manufacture whereby a circular ring of permanent magnetic segments is formed for use in NMR imaging systems.
U.S. Pat. No. 4,093,912 (Double, et al.) teaches and describes an NMR magnet assembly with pole face parallelism adjustment used to align the faces of magnetic pole pieces used in NMR magnet arrangements.
U.S. Pat. No. 5,028,903 (Aubert) teaches and describes a spherical permanent magnet with equatorial access consisting of a pair of hemispherical magnet arrangements creating therewithin an air gap for use in MRI procedures.
My co-pending U.S. patent application Ser. No. 09/388,836 teaches and describes a C-shaped MRI magnet arrangement with individual magnet elements which are separately moveable with respect to one another to adjust the field strength and uniformity of the magnetic field formed in the closed-loop apparatus.
U.S. Pat. No. 5,900,793 (Katznelson) teaches and describes a single magnet grouping intended for use in non-closed loop apparatus, having at least one magnet grouping which has toroidal magnets individually adjustable with respect to each other while preserving an opening through the magnet groupings to allow access to a patient positioned near the grouping.
It is an object of the present invention to provide a permanent magnet arrangement for use in MRI equipment in which the strength of the magnetic field created by the magnets used in the arrangement is increased without significantly increasing the weight of the arrangement.
It is a further object of the present invention to provide a field-strengthening arrangement adaptable to a wide variety of shim arrangements.
Yet another object of the present invention is to provide embodiments of the present invention that are inexpensive to construct and maintain.
An additional object of the present invention is to provide such magnetic field enhancements which take full advantage of the adjustability of the resulting magnetic field with simple mechanical expedients which do not involve moving the magnets themselves.
A permanent magnet arrangement adapted for use in MRI examinations has a generally C-shaped yoke formed from a magnetically conductive material such as soft steel. A pair of permanent magnet groupings is positioned at and joined to the open ends of the C-shaped yoke in parallel, spaced relationship to create an air gap therebetween within which a test subject is positioned to undergo MRI examination. In a preferred embodiment, each magnet arrangement consists of a disk-shaped magnet. Each magnet is attached to a backplate which, in turn, is attached to the C-shaped yoke. An adjusting mechanism formed as part of the backplate allows the magnetic field strength and uniformity to be adjusted to form a magnetically-conductive path between the magnets across the air gap and, thereby, through the yoke.
While the following describes a preferred embodiment or embodiments of the present invention, it is to be understood that this description is made by way of example only and is not intended to limit the scope of the present invention. It is expected that alterations and further modifications, as well as other and further applications of the principles of the present invention will occur to others skilled in the art to which the invention relates and, while differing from the foregoing, remain within the spirit and scope of the present invention as herein described and claimed.
These and further objects of the present invention will become more apparent upon consideration of the accompanying drawings wherein: